Food differentiation among coexistent species in the field is important strategy for copepods to acquire materials and maintain population stabilization. In situ diet analysis of co-occurring six copepod species in coral waters of the Sanya Bay was conducted using a PCR protocol based on 18S ribosomal gene. Various prey organisms were uncovered, including dinoflagellate, diatom, green algae and plant, protozoa and metazoan. All these spatially co-existing six species showed different dietary diversity, with the food niche breadth (B) ranging from 1.00 (Temora turbinate in ming) to 10.68 (Calanopia elliptica in night). While food overlap between all these copepods were low, with the average value of the diet niche overlap index being approximately 0.09. Even temporally co-existing species sampled from the same time point fed on different groups of prey items with the food overlap index of 0.04 to 0.07 in midday and night but 0 in ming. As the most important dominant copepod in the Sanya Bay, Subeucalanus subcrassus seems to be capable to regulate its feeding, by exhibiting a rhythm of herbivorous feeding in midday and civorous feeding in ming and night, to better coordinate with other competitors for utilization of food resources. For most copepods, none of the prey items belonged to the dominant phytoplankton in the ambient water, indicating that copepod can better their survival by widening the choice of potential food resources in food limited environment. The dietary separation observed here might be important strategy for copepod to maintain population stabilization and thriving in the Sanya coastal waters.